This invention relates to an apparatus and a method for non-destructively reading and selectively summing the signals at various positions along a shift register and in particular, to an apparatus and a method for reading and selectively summing the electrical signals at the taps of a delay line, for example, fabricated from a charge transfer device which is used in real time transversal filters and correlation devices.
Shift registers and delay lines provide, with varying degrees of timing control, a system for storing analog and/or digital information in a series array. When the shift register or delay line is provided with signal taps, corresponding to sampling the contents of the stored signal at selected times, there becomes available a convenient and efficient method for effecting, at high speed, such operations as correlation, filtering, etc.
Originally, efficient techniques employing delay lines were used substantially only with digital shift registers, in part because of the ease of working with and the low cost of digital circuitry. Recently however, since about 1970, controlled discrete time, continuous amplitude (analog) delay lines, charge transfer devices (CTD's), such as charge coupled devices (CCD's) and bucket brigade devices (BBD's), have become available. These devices store and transfer packets of charge under the control of externally generated clock signals and provide, at a plurality of taps spaced along the delay line, output signals which represent the stored analog signal at specified time intervals. The time intervals depend upon the device structure and the system clocking.
The tapped outputs of the charge transfer device are particularly difficult to read if the information stored therein is not to be altered during the reading procedure. Several methods have been developed, however, for non-destructively reading the tapped outputs of a charge transfer device. According to one method, used with a fixed weight tapping technique which is typical of split-electrode CCD transversal filters, the signal output at each device tap is capacitively coupled to one of two summing buses. One of the buses represents the sum of all positive weights while the second bus represents the sum of all negative weights. A differential current meter then measures the difference in charge flow, or current, which is required to maintain the buses at a fixed preset potential. That charge flow or current is a short duration pulse signal and represents the output of, for example, a transversal filter.
It is often desirable, in connection with transversal filters and correlation devices, to provide a programmable tapping network so that the weights and/or the taps along the delay line can be changed under control of a program control element. One common technique for implementing a programmable correlator comprises connecting a floating gate, which is one preferred structure for accessing the stored information, from each tap position to the gate of a separate buffer MOSFET. The current flowing through each MOSFET, which is modulated by the tapped electrical signal at the respective tapping position, is directed generally through one of two associated program controlled switches, to one of the two summing buses. Thus, each switch connects to the MOSFET output at its input and to one bus at its output. The controlled switches associated with one tapping position are closed at most one at a time and the switching function is generally implemented using additional MOSFET's. The state or states of the additional MOSFET's are controlled by a program control element. The resulting difference in total current flow on the two buses, which are maintained at the same potential, represents the correlator output.
These and other methods for reading and combining the tap outputs of a delay line or shift register each suffer several serious drawbacks. First, there is a large background or bias level which is present, at all times, in the tap output, even during a "no-signal" condition. The bias component or level is a DC offset which is present at each tap for several reasons. First, the signal appearing at each tap originates as a bipolar signal whereas there is the requirement of presenting the output of the tapping system as a unipolar variable such as charge, current, or voltage. Thus, the variable must contain an offset or bias component to allow the relative positive and negative signal excursions to remain unipolar. A second source of the bias offset is a non-zero DC component of the signal in the shift register. In addition, the prior art methods also often require an offset so that the shift register signal electrodes can be preset to a certain level to store or transfer the input signals.
While the individual sources of the offset bias can be of different polarities, the effect of the offset biases add. The result, when the tapped signals are summed on the respective buses, is a total bias level on each bus which is often many times larger than the desired time-varying summed information signal which is being detected. A fundamental problem thereby arises when the number of summed signals on each bus is unequal and can vary, because the bias levels on each bus are then also unequal and can similarly vary. The difference in the bias components of the summed signals on the two buses can be and often is interpreted as part of the time-varying small signal component by the output differencing circuitry.
In the case of a fixed weight tapping method wherein the processed signals have zero or fixed DC levels, the difference in total bias components on the two summing buses is constant. This fact allows the unwanted bias component to be "bucked out" at the output differencing circuitry although there may be practical problems in doing this when the bias difference is much greater than the desired signal as is so often the case. For programmable correlators and filters with adjustable and selectable tap weights, the problem is much more severe. Here the total bias component flowing to each bus is variable and therefore the difference in bias levels when the two buses are "summed" is also variable. This changing difference in bias level is extremely difficult to compensate for and it severely degrades the device dynamic range by placing a lower limitation on the small time-varying signals that can be observed.
Thus for example in programmable correlators used in connection with receivers employing pueudo-random codes, there is a "code-dependent bias" which varies as the numbers of ones and zeros in the reference code vary. This can affect the bias level on each of the respective summing buses and a variable bias can thus occur because bits in the reference code control at least a pair of switches which determine to which summing bus, (if any), the tapped signal (and bias) are directed. The practical effect of the code-dependent bias on present correlators is to restrict the user to codes which contain a fixed number of ones and zeros. This is a very severe limitation.
In a similar manner, when the tap weights in transversal filters are changed, one is again subject to the impractical limitation of maintaining constant the total bias component on each summing bus.
A principal object of the present invention is therefore an apparatus and method for substantially removing the limitation in programmable correlators and transversal filters, that the numbers of ones and zeros be constant or fixed. Other objects of the invention are an apparatus and method having greater dynamic range than prior art methods and apparatus, having an improved information signal to background signal ratio, and reducing the resulting bias component significantly over prior art systems. Further objects of the invention comprise a non-destructive reading apparatus and method for sensing charge in charge transfer devices, for providing the reading elements on the same semiconductor substrate, and for providing a programmable unit wherein the correlation programming can reliably be varied. Yet further objects of the invention are an apparatus and method which can be used in connection with different varying weightings, which are not limited to a substantially binary reference signal, and which can be used in connection with both transversal filters and correlation methods and devices.